The kidnap and killing of a British and Italian hostage in northern Nigeria marks a worrying new development in the violence wreaked by the country's militant Islamists.

Responsibility for the abduction was initially claimed by a previously unheard-of group called al-Qaida in the Land Beyond the Sahel, but Nigerian security sources believe that the kidnappers came from a faction of Boko Haram, an Islamist sect responsible for almost 1,000 deaths since it launched an uprising in 2009.

According to Nigerian security sources, the hostages were being kept in the remote northern city of Sokoto at the time of the attempted rescue. The centuries-old Muslim trading hub has until now escaped the brunt of violence by Boko Haram, which means "western education is sinful" in the Hausa language of northern Nigeria. But several hundred miles of porous and poorly policed borders with Niger makes Sokoto a haven for kidnappers from Boko Haram or al-Qaida in the Islamic Maghreb, known as Aqim.

Vast geographies and weak central governments mean that Nigeria and its northern neighbours have struggled to prevent ordinary citizens or criminal-minded terrorists from slipping across borders.

Roaming the vast deserts that span Niger, Mauritania, Mali, Chad and Western Sahara, Aqim operates largely unchecked by these under-resourced governments. But the Algerian-founded movement has yet to implant itself in Nigeria where militancy, to date, has been a localised phenomenon.

Officials say factions within each of the groups have been in contact with each other. According to Nigerian intelligence officials, members of the more radical Boko Haram factions have received training from Aqim in Algeria and possibly Afghanistan. Aqim is thought to have given Boko Haram advice on urban terrorist tactics and suicide bombings.

Aqim has perfected what analysts call a "kidnap economy", thriving off the abduction and ransom of westerners and Africans. It often snatches hostages in one country and moves them across one or more borders, ending up in Aqim bases in Mali. Reports suggest Chris McManus and Franco Lamolinara were moved around but remained within Nigerian borders, which makes it unlikely that Aqim was behind the atrocity.

To date, Boko Haram has shunned kidnapping as a cash cow or ideology. The group generally favours untargeted mass bomb attacks. But it has shown increasing sophistication in its campaign, graduating from crude bombs to more sophisticated improvised explosive devices. Is targets have also become more ambitious: an attack on the UN office in Abuja last year signalled that foreigners were considered legitimate targets.

The diversity of methods suggests, observers say, that the group is increasingly splintered. That raises the frightening possibility that the kidnappers are most likely an offshoot of Boko Haram mimicking Aqim's tactics. This could spell a new chapter of terror in the north of the country, mirroring the decade of kidnappings that plagued Nigeria's southern oil creeks until a 2009 amnesty. Most hostages taken by oil militants were held for ransom and released safely – but the jihadi ideology that fuels Islamist militancy would make such outcomes far less likely in the north.

A British construction worker and his Italian colleague have been murdered by terrorists in Nigeria as UK special forces, supported by the Nigerian army, attempted to rescue them. Find out how it happened ...